Zoom lens system

Abstract

A zoom lens system composed, in order from the object side, of a first positive lens unit and a second negative lens unit, and configured to change a magnification thereof by varying an airspace reserved between the first lens unit and the second lens unit. The first lens unit is composed, in order from the object side, of a first negative lens element, a second plastic lens element which has at least one aspherical surface and a remarkably weak refractive power, and a cemented lens component consisting of a third negative lens element and a fourth positive lens element.

I claim: 
     
       1. A zoom lens system comprising, in order from an object side: a first lens unit having a positive refractive power; and   a second lens unit having a negative refractive power,   wherein said zoom lens system is configured to change a magnification thereof by varying an airspace reserved between said first lens unit and said second lens unit,   said first lens unit comprises, in order from the object side, of a first negative lens element, a second plastic lens element which has at least one aspherical surface and a weak refractive power, and a cemented lens component consisting of a third negative lens element and a fourth positive lens element, and   said first lens unit satisfies the following conditions (1) and (2):   40<ν1<75                                                (1)       40<ν2<95                                                (2)     wherein the reference symbols ν1 and ν2 represent Abbe's numbers of said first lens element and said second lens element respectively.     
     
     
       2. A zoom lens system according to claim 1 satisfying the following condition (3):   |f.sub.1 /(f.sub.L2 ·z)|<0.07   (3)     wherein the reference symbol f 1  represents a focal length of said first lens unit, the reference symbol f L2  designates a focal length of said second lens element and the reference symbol z denotes a vari-focal ratio.   
     
     
       3. A zoom lens system according to claim 1 or 2 satisfying the following condition (4):   |f.sub.T ·(n.sub.2 -1) 1/r.sub.3 (P)-1/r.sub.4 (P)!|<0.60                                       (4)     wherein the reference symbol f T  represents a focal length of said zoom lens system as a whole at a tele position, the reference symbol n 2  designates a refractive index of said second lens element for the d-line, and the reference symbols r 3  (P) and r 4  (P) denote local radii of curvature on an object side surface and an image side surface respectively of said second lens element which are given by the following equations:     r.sub.3 (P)=Yt.sub.3 /sin φ.sub.3       r.sub.4 (P)=Yt.sub.4 /sin φ.sub.4     wherein the reference symbols Yt 3  and Yt 4  represent heights of an axial marginal ray on the object side surface and the image side surface respectively of said second lens element at a tele position, and the reference symbols φ 3  and φ 4  designate values which are given by the equations shown below:     φ.sub.3 =tan.sup.-1  Yt.sub.3 /r.sub.3 (Q){1-(K.sub.3 +1)Yt.sub.3.sup.2 /r.sub.3 (Q).sup.2 }.sup.-1/2 +4A.sub.34 Yt.sub.3.sup.3 +6A.sub.36 Yt.sub.3.sup.5 +8A.sub.38 Yt.sub.3.sup.7 + . . . !       φ.sub.4 =tan.sup.-1  Yt.sub.4 /r.sub.4 (Q){1-(K.sub.4 +1)Yt.sub.4.sup.2 /r.sub.4 (Q).sup.2 }.sup.-1/2 +4A.sub.44 Yt.sub.4.sup.3 +6A.sub.46 Yt.sub.4.sup.5 +8A.sub.48 Yt.sub.4.sup.7 + . . . !     wherein the reference symbols r 3  (Q) and r 4  (Q) represent axial radii of curvature on the object side surface and the image side surface respectively of said second lens element, the reference symbols K 3  and K 4  designate conical constants, and the reference symbols A 34 , A 36 , A 38 , . . . and A 44 , A 46 , A 48 , . . . denote aspherical surface coefficients.   
     
     
       4. A zoom lens system according to claim 1 satisfying the following condition (6):   0.35<d.sub.6 /f.sub.L4 <0.85                               (6)     wherein the reference symbol f L4  represents a focal length of said fourth lens element and the reference symbol d 6  designates a thickness of said fourth lens element.   
     
     
       5. A zoom lens system according to claim 1 wherein said second lens unit is composed, in order from the object side, of a single positive lens element and a single negative lens element. 
     
     
       6. A zoom lens system according to claim 1 satisfying the following condition (7):   -4.5<f.sub.L5 /f.sub.2 <-1.5                               (7)     wherein the reference symbol f L5  represents a focal length of said fifth lens element and the reference symbol f 2  designates a focal length of said second lens unit.   
     
     
       7. A zoom lens system according to claim 1 wherein an aspherical surface is used as an object side surface of the positive lens element disposed in said second lens unit. 
     
     
       8. A zoom lens system according to claim 1 satisfying the following condition (5):   1.55<n.sub.4 <1.75                                         (5)     wherein the reference symbol n 4  represents a refractive index of said fourth lens element.   
     
     
       9. A zoom lens system comprising, in order from an object side: a first lens unit having a positive refractive power; and   a second lens unit having a negative refractive power, wherein said zoom lens system is configured to change a magnification thereof by varying an airspace reserved between said first lens unit and said second lens unit,   said first lens unit comprises, in order from the object side, a first negative meniscus lens element having a convex surface on the object side, a second lens element which is made of a plastic material, and has a convex surface on the object side, at least one aspherical surface and a weak refractive power, a cemented lens component consisting of a third negative meniscus lens element having a convex surface on the object side and a fourth positive biconvex lens element,   said second lens unit is composed of a fifth positive meniscus lens element which has at least one aspherical surface and a convex surface on the image side, and a sixth negative meniscus lens element which has a convex surface on the image side, and   said first lens unit satisfies the following conditions (1) and (2):   40<ν.sub.1 <75                                          (1)       40<ν.sub.2 <95                                          (2)     wherein the reference symbols ν 1  and ν 2  represent Abbe's numbers of said first lens element and said second lens element, respectively.     
     
     
       10. A zoom lens system comprising, in order from an object side: a first lens unit having a positive refractive power; and   a second lens unit having a negative refractive power,   wherein said zoom lens system is configured to change a magnification thereof by varying an airspace reserved between said first lens unit and said second lens unit,   said first lens unit is composed, in order from the object side, of a first negative biconcave lens element, a second lens element which is made of a plastic material, and has a convex surface on the object side, at least one aspherical surface and a weak refractive power, and a cemented lens component consisting of a third negative meniscus lens element having a convex surface on the object side and a fourth positive biconvex lens element,   said second lens unit is composed of a fifth positive meniscus lens element which has at least one aspherical surface and a convex surface on the image side, and a sixth negative meniscus lens element which has a convex surface on the image side, and wherein said first lens unit satisfies the following conditions (1) and (2);   40<ν.sub.1 <75                                          (1)       40<ν.sub.2 <95                                          (2)     wherein the reference symbols ν 1  and ν 2  represent Abbe's numbers of said first lens element and said second lens element, respectively.     
     
     
       11. A zoom lens system according to claim 9 wherein said fifth lens element has an aspherical surface on the object side. 
     
     
       12. A zoom lens system according to claim 10 wherein said fifth lens element has an aspherical surface on the object side. 
     
     
       13. A zoom lens system according to claim 11 wherein said second lens element has an aspherical surface on the object side. 
     
     
       14. A zoom lens system according to claim 12 wherein said second lens element has an aspherical surface on the object side. 
     
     
       15. A zoom lens system according to claim 9 satisfying the following condition (3):   |f.sub.1 /(f.sub.L2 ·z)|<0.07   (3)     wherein the reference symbol f 1  represents a focal length of said first lens unit, the reference symbol f L2  designates a focal length of said second lens element and the reference symbol z denotes a vari-focal ratio.   
     
     
       16. A zoom lens system according to claim 9 or 15 satisfying the following condition (4):   |f.sub.T ·(n.sub.2 -1) 1/r.sub.3 (P)-1/r.sub.4 (P)!|<0.60                                       (4)     wherein the reference symbol f T  represents a focal length of said zoom lens system as a whole at the tele position, the reference symbol n 2  designates a refractive index of said second lens element for the d-line, and the reference symbols r 3  (P) and r 4  (P) denote local radii of curvature on an object side surface and an image side surface respectively of said second lens element which are given by the following equations:     r.sub.3 (P)=Yt.sub.3 /sin φ.sub.3       r.sub.4 (P)=Yt.sub.4 /sin φ.sub.4     wherein the reference symbols Yt 3  and Yt 4  represent heights of an axial marginal ray on the object side surface and the image side surface respectively of said second lens element at a tele position, and the reference symbols φ 3  and φ 4  designate values which are given by the equations shown below:     φ.sub.3 =tan.sup.-1  Yt.sub.3 /r.sub.3 (Q){1-(K.sub.3 +1)Yt.sub.3.sup.2 /r.sub.3 (Q).sup.2 }.sup.-1/2 +4A.sub.34 Yt.sub.3.sup.3 +6A.sub.36 Yt.sub.3.sup.5 +8A.sub.38 Yt.sub.3.sup.7 + . . . !       φ.sub.4 =tan.sup.-1  Yt.sub.4 /r.sub.4 (Q){1-(K.sub.4 +1)Yt.sub.4.sup.2 /r.sub.4 (Q).sup.2 }.sup.-1/2 +4A.sub.44 Yt.sub.4.sup.3 +6A.sub.46 Yt.sub.4.sup.5 +8A.sub.48 Yt.sub.4.sup.7 + . . . !     wherein the reference symbols r 3  (Q) and r 4  (Q) represent axial radii of curvature on the object side surface and the image side surface respectively of said second lens element, the reference symbols K 3  and K 4  designate conical constants, and the reference symbols A 34 , A 36 , A 38 , . . . and A 44 , A 46 , A 48 , . . . denote aspherical surface coefficients.   
     
     
       17. A zoom lens system according to claim 9 satisfying the following condition (6)   0.35<d.sub.6 /f.sub.L4 <0.85                               (6)     wherein the reference symbol f L4  represents a focal length of said fourth lens element and the reference symbol d 6  designates a thickness of said fourth lens element.   
     
     
       18. A zoom lens system according to claim 9 satisfying the following condition (7):   -4.5<f.sub.L5 /f.sub.2 <-1.5                               (7)     wherein the reference symbol f L5  represents a focal length of said fifth lens element and the reference symbol f 2  designates a focal length of said second lens unit.   
     
     
       19. A zoom lens system according to claim 10 satisfying the following condition (3):   |f.sub.1 /(f.sub.L2 ·z)|<0.07   (3)     wherein the reference symbol f 1  represents a focal length of said first lens unit, the reference symbol f L2  designates a focal length of said second lens element and the reference symbol z denotes a vari-focal ratio.   
     
     
       20. A zoom lens system according to claim 10 satisfying the following condition (4):   |f.sub.T ·(n.sub.2 -1) 1/r.sub.3 (P)-1/r.sub.4 (P)!|<0.60                                       (4)     wherein the reference symbol f T  represents a focal length of said zoom lens system as a whole at the tele position, the reference symbol n 2  designates a refractive index of said second lens element for the d-line, and the reference symbols r 3  (P) and r 4  (P) denote local radii of curvature on an object side surface and an image side surface respectively of said second lens elements which are given by the equations shown below:     r.sub.3 (P)=Yt.sub.3 /sin φ.sub.3       r.sub.4 (P)=Yt.sub.4 /sin φ.sub.4     wherein the reference symbols Yt 3  and Yt 4  represent heights of an axial marginal ray on the object side surface and the image side surface respectively of said second lens element at a tele position, and the reference symbols φ 3  and φ 4  designate values which are given by the equations shown below:     φ.sub.3 =tan.sup.-1  Yt.sub.3 /r.sub.3 (Q){1-(K.sub.3 +1)Yt.sub.3.sup.2 /r.sub.3 (Q).sup.2 }.sup.-1/2 +4A.sub.34 Yt.sub.3.sup.3 +6A.sub.36 Yt.sub.3.sup.5 +8A.sub.38 Yt.sub.3.sup.7 + . . . !       φ.sub.4 =tan.sup.-1  Yt.sub.4 /r.sub.4 (Q){1-(K.sub.4 +1)Yt.sub.4.sup.2 /r.sub.4 (Q).sup.2 }.sup.-1/2 +4A.sub.44 Yt.sub.4.sup.3 +6A.sub.46 Yt.sub.4.sup.5 +8A.sub.48 Yt.sub.4.sup.7 + . . . !     wherein the reference symbols r 3  (Q) and r 4  (Q) represent axial radii of curvature on the object side surface and the image side surface respectively of said second lens element, the reference symbols K 3  and K 4  designate conical constants, and the reference symbols A 34 , A 36 , A 38 , . . . and A 44 , A 46 , A 48 , . . . denote aspherical surface coefficients.   
     
     
       21. A zoom lens system comprising, in order from an object side: a first lens unit having a positive refractive power; and   a second lens unit having a negative refractive power,   wherein said zoom lens system is configured to change a magnification thereof by varying an airspace reserved between said first lens unit and said second lens unit,   said first lens unit comprises, in order from the object side, a first negative biconcave lens element, a second lens element which is made of a plastic material, and has a convex surface on the object side, at lest one aspherical surface and a weak refractive power, and a cemented lens component consisting of a third negative meniscus lens element having a convex surface on the object side and a fourth positive biconvex lens element,   said second lens unit is composed of a fifth positive meniscus lens element which has at least one aspherical surface and a convex surface on the image side, and a sixth negative meniscus lens element which has a convex surface on the image side, and   said first lens unit satisfies the following conditions (1) and (2):   40<ν.sub.1 <75                                          (1)       40<ν.sub.2 <95                                          (2)     wherein the reference symbols ν 1  and ν 2  represent Abbe's numbers of said first lens element and said second lens element, respectively, and satisfying the following condition (6):     0.35<d.sub.6 /f.sub.L4 <0.85                               (6)     wherein the reference symbol f L4  represents a focal length of said fourth lens element and the reference symbol d 6  designates a thickness of said fourth lens element.     
     
     
       22. A zoom lens system according to claim 10 satisfying the following condition (7):   -4.5<f.sub.L5 /f.sub.2 <-1.5                               (7)     wherein the reference symbol f L5  represents a focal length of said fifth lens element and the reference symbol f 2  designates a focal length of said second lens unit.   
     
     
       23. A zoom lens system according to claim 9 or 10 satisfying the following condition (5):   1.55<n.sub.4 <1.75                                         (5)     wherein the reference symbol n 4  represents a refractive index of the fourth lens element for the d-line.   
     
     
       24. A zoom lens system according to claim 1, 9 or 10, wherein said first lens element is made of glass.